Roger Federer and Rafael Nadal: The greatest tennis rivalry of all timeReuters

Rafael Nadal will become the new men's world number one next week after Roger Federer opted to withdraw from the Cincinnati Masters with a back injury. The 19-time grand slam champion was the only player able to deny the Spaniard at the upcoming Masters 1000 event in Ohio, but his decision to pull out will see Nadal return to the summit of the ATP standings when they are re-calculated on 21 August.

Andy Murray was already assured of being replaced as world number one after deciding against playing at last week's Rogers Cup and in Cincinnati, but will drop no lower than to number two ahead of the US Open. Nadal meanwhile recaptures the ranking after a three-year absence dating back to July 2014 and is set to build on his 141 weeks in top spot.

Thirty-six-year-old Federer meanwhile picked up a back injury in his two-set loss to Alexander Zverev in the Rogers Cup final in Montreal and must now be considered a doubt for the final grand slam of the year at Flushing Meadows. The Swiss had pledged to play the remainder of the year, having missed the entire clay court season, but this latest blow will raise concerns over his next competitive appearance.

Federer had complained of "muscle pain" following his loss to Zverev but refrained from specifying or indeed revealing that he had picked up an injury. Reports suggest Federer did inform Zverev during the match that he was suffering from a back problem, despite deciding to keep the development from the media.

"I am very sorry to pull out of the Western & Southern Open as I always enjoy playing here," Federer said in a statement. "Cincinnati has some of the best fans in the world and I am sorry I will miss them. Unfortunately, I tweaked my back in Montreal and I need to rest this week." Lucky loser Thomas Fabbiano of Italy will replace him in the draw.

In Federer's absence, Nadal has the chance to extend his lead over both him and Murray as he attempts to end 2017 as the year-end number one. The Spaniard will face either Richard Gasquet or John Patrick-Smith in the second round in Cincinnati, while the likes of Giles Muller, Jo-Wilfried Tsonga and Dominic Thiem stand between him and a just a second title at the event, and first since 2013.

Nadal is back on top of men's tennis after a three-year absence.Getty Images